Numerous studies have implicated small soluble oligomeric aggregates of Aβ as toxic species in Alzheimer's disease (AD), and increasing evidence also implicates oligomeric forms of tau as having a direct role in disease pathogenesis of AD and other tauopathies such as Frontotemporal Dementia (FTD). As the focus of Aβ studies has slowly shifted toward soluble Aβ species and mechanisms, new reagents were needed that could specifically identify the variety of different aggregate species present. Indeed, many contradictory studies on the role of Aβ aggregation in AD were reported and progress impeded because suitably selective reagents were not available to characterize the aggregate species present. Increasing evidence from cell and animal models indicate that oligomeric rather than fibrillar forms of tau are toxic and correlate with neuronal degeneration, therefore well characterized reagents that can specifically recognize the diversity of tau morphologies present in the human brain are critically needed to facilitate studies to identify the most promising tau species for use as biomarkers of disease and to study toxic mechanisms.
The microtubule associating protein tau is a major component of the neurofibrillary tangles associated with AD and tauopathies that are characterized by hyperphosphorylation and aggregation of tau. Tau plays an important role in assembly and stabilization of microtubules. Tau is a natively unfolded protein, and similar to a number of other natively unfolded proteins, it can aberrantly fold into various aggregate morphologies including β-sheet rich fibrillar forms. The different types of post-translational modifications of tau in AD include phosphorylation, glycosylation, glycation, prolyl-isomerization, cleavage or truncation, nitration, polyamination, ubiquitination, sumoylation, oxidation and aggregation. Tau has 85 putative phosphorylation sites, and excess phosphorylation can interfere with microtubule assembly. Tau can be modified by phosphorylation or by reactive nitrogen and oxygen species among others. Elevated total tau concentration in CSF has been correlated with AD, as has the presence of various phosphorylated tau forms, and the ratio of tau to Aβ42. Reactive nitrogen and oxygen can modify tau facilitating formation of aggregate forms including oligomeric species. Levels of oligomeric tau have also been implicated as a potential early diagnostic for AD. Therefore, determination of total tau, phosphorylated tau and oligomeric tau concentrations all have potential value as diagnostics for neurodegenerative diseases including tauopathies and AD.
Tau is an intrinsically unstructured protein due to its very low hydrophobic content containing a projection domain, a basic proline-rich region, and an assembly domain. Hexapeptide motifs in repeat regions of tau give the protein a propensity to form β-sheet structures which facilitate interaction with tubulin to form microtubules as well as self-interaction to form pathological aggregates such as paired helical filaments (PHF). Hyperphosphorylation of tau, particularly in the assembly domain, decreases the affinity of tau to the microtubules and impairs its ability to regulate microtubule dynamics and axonal transport. In addition, parts of the basic proline-rich domain and the pseudo-repeat also stabilize microtubules by interacting with its negatively charged surface. Alternative splicing of the second, third and tenth exons of tau results in six tau isoforms of varying length in the CNS. The assembly domain in the carboxyl-terminal portion of the protein contains either three or four repeats (3R or 4R) of a conserved tubulin-binding motif depending on alternative splicing of exon 10. Tau 4R isoforms have greater microtubule binding and stabilizing ability than the 3R isoforms. Human adult brains have similar levels of 3R and 4R isoforms, whereas only 3R tau is expressed at the fetal stage. In tauopathies, mutations altering the splicing of tau transcript and the ratio of 3R to 4R tau isoforms are sufficient to cause neurodegenerative disease. Therefore tau in human brain tissue can exist in a variety of different lengths and morphologies and with multiple post-translational modifications.
Tau plays a critical role in the pathogenesis of AD and studies show that reduction of tau levels in AD animal models reverses disease phenotypes and that tau is necessary for the development of cognitive deficits in AD models caused by over-expression of Aβ. While NFTs have been implicated in mediating neurodegeneration in AD and tauopathies, animal models of tauopathy have shown that memory impairment and neuron loss do not associate well with accumulation of NFT. Animal studies showed improvement in memory and reduction in neuron loss despite the accumulation of NFTs, a regional dissociation of neuron loss and NFT pathology, and hippocampal synapse loss and dysfunction and microglial activation months before the accumulation of filamentous tau inclusions. The pathological structures of tau most closely associated with AD progression are tau oligomers. All these studies suggest that tau tangles are not acutely neurotoxic, but rather that pretangle oligomeric tau species are responsible for the neurodegenerative phenotype, similar to toxic role of oligomeric Aβ species.
Numerous studies suggest that extracellular tau species contribute to neurotoxicity through an “infectious” model of disease progression. For example, tau pathology spreads contiguously throughout the brain from early to late stage disease, extracellular tau aggregates can propagate tau misfolding from the outside to the inside of a cell, brain extract from a transgenic mouse with aggregated mutant human tau transmits tau pathology throughout the brain in mice expressing normal human tau, induction of pro-aggregation human tau induces formation of tau aggregates and tangles composed of both human and normal murine tau (co-aggregation), and levels of tau rise in CSF in AD, whereas Aβ levels decrease. A receptor-mediated mechanism for the spread of tau pathology by extracellular tau has been described.
Collectively, these studies all indicate that aggregated oligomeric species of tau, both intracellular and extracellular are vitally important in AD and other tauopathies. In order to more clearly define the role of individual tau forms in disease, there is a critical need to develop a series of well-defined reagents that selectively recognize individual target morphologies, and to use these reagents to identify which tau forms are the best biomarkers for AD, which forms are involved in toxicity both intra- and extracellularly, and which forms in brain tissue and CSF samples can distinguish between healthy and AD patients.
Therefore, reagents that can specifically target tau oligomers would be valuable tools for diagnostic and therapeutic applications for AD, frontotemporal dementia, other tauopathies and neurodegeneration following traumatic brain injury.
Accordingly, there exists the need for new therapies and reagents for the treatment of Alzheimer's disease, frontotemporal dementia, other tauopathies and neurodegeneration following traumatic brain injury, in particular, therapies and reagents capable of effecting a therapeutic and diagnostic benefit at physiologic (e.g., non-toxic) doses.